The use of stationary so-called “gas suspension” calciners, in an alumina producing process in which particles are transported through drying, calcination and cooling zones is well known in the art. The resulting product may be separated by gravity, or by cycloning or filtration. The main components of a system to be used in such a process are a Venturi drier, a multi-stage cyclone preheater, a gas-suspension calciner, a multi-stage cyclone cooler, a secondary fluid-bed cooler, and a dust removal and recycling system which incorporates an electrostatic precipitator. A wet hydrate filter cake is fed to the flash drier and the dried material is then passed through different stages of the system. In such an apparatus the heated process gas moves in a general stage wise counter-current relationship to the material path, and typically moves through the cooler, the suspension calciner, the preheating/precalcining zone and finally to the dust removal and recycling system.
In an alumina manufacturing process undesirable organic materials are produced. These organics include, for example, benzene derivatives, aldehydes, and ketones. Such organic compounds are foul-smelling and typically originate from organic filter aids added to the hydrate filter to lower the moisture in the hydrate fed to the calciner or from organics dissolved in liquor pockets trapped in, or adsorbed to the hydrate crystals forming the hydrate particles as they are produced in the precipitation plant. Whatever the source, such organics never reach the gas suspension calciner, but are volatized in the 300-350° C. temperature range or lower, depending upon the volatility of the organics, during the preheating stage of the hydrate particles. The resulting gaseous volatile organic compounds exit the preheating chamber with the process gas and contribute to the odor in stack gases.
It is an object of the present invention, therefore, to effectively reduce the volatile, foul-smelling organics produced in a gas suspension calciner process utilized to manufacture alumina.